1. Field of the Invention
The present invention relates to a free layer for a spin valve sensor with lower uniaxial anisotropy field and a method of making and, more particularly, to a free layer with employs a combination of nickel iron (NiFe) and cobalt iron (CoFe) films.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a nonmagnetic gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic field into the pole pieces that fringes across the gap between the pole pieces at the ABS. The fringe field writes information in the form of the aforementioned magnetic impressions in circular tracks on the rotating disk.
An exemplary high performance read head employs a spin valve sensor for sensing magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) which is an exposed surface of the sensor that faces the rotating disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers.
The sensitivity of the sensor is quantified as magnetoresistance or magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. A spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. The sensitivity of a spin valve sensor depends upon the response of the free layer to signal fields from a rotating magnetic disk. The magnetic moment of the free layer depends upon the material or materials employed for the free layer. As the magnetic moment of the free layer increases the responsiveness of the free layer decreases. This means that for a given signal field from the rotating magnetic disk the magnetic moment of the free layer will not rotate as far from its parallel position to the ABS which causes a reduction in signal output.
In order to improve the sensitivity of the spin valve sensor a soft magnetic material, such as nickel iron (NiFe), is employed for the free layer. It has been found, however, that when the free layer employs a cobalt based film in addition to the nickel iron (NiFe) film that the magnetoresistive coefficient dr/R increases when the cobalt based film is located between and interfaces the nickel iron (NiFe) film and the copper (Cu) spacer layer. A cobalt based film, such as cobalt (Co) or cobalt iron (CoFe), has a magnetic moment of approximately 1.7 times the magnetic moment of nickel iron (NiFe) for a given thickness. The addition of a cobalt or cobalt based film increases the stiffness (uniaxial anisotropy field HK) of the free layer in its response to signal fields and reduces the sensitivity of the spin valve sensor. Uniaxial anisotropy field is the amount field required to rotate the magnetic moment of the free layer from a position pararallel to the ABS to a position perpendicular thereto. Further, the cobalt based material causes the free layer structure to have a hysteresis. This hysteresis is indicated in a hysteresis loop which is a graph of the magnetic moment M of the free layer in response to an applied field H (signal field) directed perpendicular to the ABS. The hysteresis loop, which is referred to as the hard axis loop, has an opening due to the hysteresis which can be on the order of 5 to 7 oersteds. The opening in the hard axis loop is quantified as hard axis coercivity HC which is measured from the origin of the x and y axes to the intersection of the loop with the x axis (applied signal). It has been found that when the hard axis coercivity is high the head generates Barkhausen noise which is due to the fact that the magnetic domains of the cobalt based layer are oriented in different directions. Accordingly, as the signal fields rotate the magnetic moment of the free layer some of the magnetic domains do not follow the directions of the signal fields. The magnetic domains that do not readily follow the signal field direction follow behind the signal field direction in an erratic behavior, referred to as jumps in their movements, which causes the aforementioned Barkhausen noise. This Barkhausen noise is superimposed upon the playback signal which is unacceptable.
In order to keep the hard axis coercivity at an acceptable low level, very thin cobalt based films can be employed, such as 2 xc3x85 thick. While a 2 xc3x85 thick cobalt based layer produces some improvement in the magnetoresistive coefficient dr/R, it has been found that thicker cobalt based films will further increase the magnetoresistive coefficient dr/R. Considering all factors, including sense current shunting, a cobalt based layer on the order of 15 xc3x85 produces the highest magnetoresistive coefficient dr/R. Unfortunately, a cobalt based layer of this thickness causes the free layer structure to have a hard axis coercivity which unacceptably reduces the sensitivity of the read head to signal fields and produces Barkhausen noise. Accordingly, it would be desirable if cobalt based films thicker than 2 xc3x85 could be employed in a free layer structure without the aforementioned problems of responsiveness to signal fields and the production of Barkhausen noise. If the hysteresis or opening in the hard axis loop is eliminated the aforementioned moment versus applied field graph (M/H graph) of the responsiveness of the spin valve sensor would be simply a straight line. This straight line, which is the transfer curve of the read head, indicates that the read head will be magnetically stable upon the application of the signal fields.
In a read head application, the uniaxial anisotropy field and the hard axis loop of the free layer structure has to be determined after the free layer is subjected to annealing at a high temperature for a period of time. This is due to the fact that during the fabrication of a read/write head combination the aforementioned first, second and third insulation layers are baked photoresist. After spinning a photoresist layer onto a wafer substrate and patterning it, the photoresist layer is annealed at a temperature of approximately 232xc2x0 C. for a period of 6 hours. Accordingly, uniaxial anisotropy field and the hard axis loop for a free layer in a read head that is combined with a write head has meaning only after this annealing. The annealing is performed in the presence of a magnetic field which is oriented perpendicular to the ABS. This orientation is 90xc2x0 to the desired orientation of the magnetic moment of the free layer. The orientation of the magnetic field perpendicular to the ABS is for the purpose of maintaining the orientation of the magnetic spins of the pinning layer. If the annealing temperature exceeds or is close to the blocking temperature of the materials employed for pinning layers the magnetic spins of the pinning material is free to move in response to any extraneous field. Any such movement would shift the magnetic moment of the pinned layer from the perpendicular position after the annealing is terminated which would destroy or seriously degrade the spin valve sensor. Unfortunately, the aforementioned annealing process increases the uniaxial anisotropy field and the coercivity making the free layer less responsive to signal fields.
The present invention provides a free layer for a spin valve sensor which has combined nickel iron (NiFe) and cobalt iron (CoFe) films with a lower uniaxial anisotropy field HK and a lower hard axis coercivity HCH after annealing than prior art such free layers after annealing. This has been accomplished by providing the cobalt iron (CoFe) film with a magnetic easy axis that is oriented perpendicular to the ABS and a nickel iron (NiFe) film with an easy axis that is oriented parallel to the ABS and parallel to the major planes of the thin film layers. The orientation of the easy axis of the nickel iron (NiFe) film is accomplished by sputter depositing nickel iron (NiFe) material in the presence of a field which is oriented parallel to the ABS and parallel to the major planes of the thin film surfaces. In a like manner, orientation of the easy axis of the cobalt iron (CoFe) is accomplished by sputter depositing cobalt iron (CoFe) in the presence of a field which is oriented perpendicular to the ABS. Surprisingly, after the sputter deposition, the uniaxial anisotropy field HK and the hard axis coercivity HCH of the free layer with the combined films are lowered by annealing the free layer at a high temperature in the presence of a field which is oriented perpendicular to the ABS. The annealing temperature was 232xc2x0 C., which is the baking temperature used to harden photoresist layers of an insulation stack in the write head, in the presence of a field of 1,000 Oe, which is the field used to maintain orientation of the magnetic spins of the pinning layer in the spin valve sensor, for a period of 8 hours. After annealing the free layer the uniaxial anisotropy field HK was 5.3 Oe as compared to 9.37 Oe for a prior art annealed free layer and the hard axis coercivity HCH was 0.57 Oe as compared to 0.95 Oe for the prior art annealed free layer.
An object of the present invention is to provide a free layer, with a cobalt based film, which has improved uniaxial anisotropy field.
Another object is to provide a free layer, with a cobalt based film, which has improved uniaxial anisotropy field after annealing in presence of a magnetic field.
A further object is to provide a free layer, with nickel iron (NiFe) and cobalt iron (CoFe) films, which has improved uniaxial anisotropy field HK and improved hard axis coercivity HCH before and after annealing at a temperature sufficient to bake photoresist in an insulation stack of a write head in the presence of a magnetic field which is oriented perpendicular to the ABS in order to maintain orientation of the magnetic moment of a pinned layer in a spin valve sensor.
Still another object is to provide various methods of making the aforementioned spin valve sensors.
Other objects and advantages of the invention will become apparent upon reading the following description taken together with the accompanying drawings.